mangroverehabilitationonhighlyerodedcoastalshorelines...

Hindawi Publishing CorporationInternational Journal of EcologyVolume 2012, Article ID 171876, 11 pagesdoi:10.1155/2012/171876

Research Article

Mangrove Rehabilitation on Highly Eroded Coastal Shorelinesat Samut Sakhon, Thailand

Matsui Naohiro,1 Songsangjinda Putth,2 and Morimune Keiyo3

1 Department of Environment, Kanso Technos Co., Ltd., Osaka 541-0052, Japan2 Fishery Department, Trang Coastal Aquaculture Station, Trang 92150, Thailand3 Power Engineering R&D Center, The Kansai Electric Power Co., Inc., Kyoto 609-0237, Japan

Correspondence should be addressed to Matsui Naohiro, matui [email protected]

Received 8 July 2011; Revised 20 September 2011; Accepted 20 October 2011

Academic Editor: L. M. Chu

Copyright © 2012 Matsui Naohiro et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The study site is currently retreating at a rate of 20 m y−1 due to severe coastal erosion and found to be highly polluted as revealedfrom the water, sediment and biological analysis. In an attempt to prevent coastal erosion, 14,000 Rhizophora mucronata (RM) treeswere planted across a heavily eroded shoreline at Samut Sakhon, Thailand. The survival rate of RM was high at the landward areaand decreased at the offshore area. The most landward plot showed the highest survival rate when measured 4 years after planting(63.5%), while only 26.7% of trees survived at the most offshore plot. NPK and coconut fiber were shown to be significantlyeffective to enhance initial tree growths in heavily eroded area.

1. Introduction

Coastal erosion is one of the most severe environmentalproblems currently affecting Thailand, and the Thai Govern-ment has designated this problem as a high priority amongnational environmental problems to be solved urgently.Coastal erosion is accelerated with the destruction of man-grove forests that normally provide protection from erosion.Thousands of abandoned shrimp ponds in coastal areas ofThailand represent decades of mangrove destruction. It istherefore necessary to rehabilitate these sites in the interestsof coastal protection.

Samut Sakhon is located approximately 50 km fromBangkok. Because of its proximity to the capital, coastal de-velopment began at this site in the 1970s. As there were nolegislative restrictions on coastal development at that time,poorly planned coastal developments were carried out up tothe very edge of the coast; this practice is now prohibited bylaw. Inland damming at the upper reaches also was activelyconducted. Sediment is transported from the upper reach tolower lands and accumulates there. Reduced flux of sedimentdue to hydraulic dam had severe impacts on changes of

Samut Sakhon coastlines. Under the influence of direct wave/wind attack and without protection from mangrove forests,unprecedented coastal erosion has occurred in this region.

Mangrove plantations in vulnerable areas could be ben-eficial for long-term coastal protection both to continuouserosion and to severe hazard such as tsunami. The presenceof mangrove as well as continuous riverine sediment flux isessential to maintain coastal stability. Over the years, a num-ber of mangrove planting projects have been carried out torestore degraded mangrove forests [1–3], some of whichaimed at protecting shoreline against storm and cyclonedamage. Rehabilitation must be urgently implemented toprevent further coastal erosion; however, planting mangroveswithin eroded areas is far more difficult than planting atother sites because of the harsh physical conditions. Quitelow survival rates of planted mangroves at the shoreline areawere reported at the Visayas [4] and at other areas in the Phil-ippines [5], as well as at Surat Thani [6] and at Samuth Song-kram in Thailand [7]. Mangrove plantation in shoreline arealeads to rapid accretion because the mangrove roots andpneumatophores effectively slow water movement and act asefficient sediment trappers [8]. Geomorphological process

2 International Journal of Ecology

affects greatly this function of mangrove vegetation. How-ever, mangroves explicitly promote sediment deposition,stimulate soil stability, and protect shoreline from erosion [3,9–11]. The success of rehabilitation efforts largely dependson the early establishment of planted mangroves. Afterunderstanding the ecology of sites and adequate species askey issues for successful mangrove plantation, we shall headfor the next step to enhance initial growth rates. Effects ofnitrogen fertilizer on mangrove growths were identified inFlorida, USA at the abandoned impoundment [12]. AlsoMatsui et al. (2010) revealed an importance of soil carbon ininitial stage of mangrove growths [13]. Taking these into theaccount, soil amendment application will be effective in en-suring rapid initial growth. There is however still limitednumber of mangrove planting by the application of soilamendment, especially at shoreline area.

The objectives of this study are therefore: (1) to under-stand the growth characteristics of mangroves species plant-ed upon highly eroded shorelines, (2) to assess the effec-tiveness of different soil amendments in promoting initialmangrove growth, and (3) to examine the pollution level ofthe study site from the viewpoints of water qualities andbiological properties to determine the influence of the sur-rounding areas on the mangrove area.

2. Methods

2.1. Study Site. The study was conducted at an abandonedshrimp pond located close to the shoreline within the Sam-ut Sakhon subdistrict/district, Samut Sakhon province,Thailand (13◦28′N, 100◦13′E) (Figure 1). Annual tidal rangemonitored at Tha Chin station (Samut Sakhon province) isabout 0.36–3.46 m above mean sea level. The wave heightrises up as high as 4 m at monsoon seasons.

Most of the mangroves disappeared from the study sitebut some species such as Avicennia marina and Rhizophoramucronata remained to grow in the edges along the road andof the former shrimp ponds. Exploitation of the area beganwith the construction of saltpans in the 1970s, followed byshrimp farming in the 1980s and associated clear-felling ofthe majority of mangrove forests. In addition to the destruc-tion of mangroves in the area, a drastic decline was reportedin fishery yields. Local fishermen reported that greater num-bers of cockles and catfish were harvested prior to the large-scale conversion of mangroves to shrimp ponds. Today, fewcatfish are harvested in the area.

2.2. Outline of Coastal Erosion at Samut Sakhon. The locationof the shoreline is generally determined by the dynamicbalance between sedimentation and erosion. Accordingly,changes in sedimentation patterns affect the dynamic balanceof the shoreline. At Nakhon Sawan (Figure 1), sedimentloading was monitored for 33 years from 1960 to 1993 [14].Figure 2 shows the changes in total sediment loading duringthis period. Sediment loading was variable during the periodup to 1970, but subsequent to this time the total amount ofsediment has gradually decreased, falling below 1011 metrictons per year for every year after 1976 except 1987. The

Bhumibol Dam was constructed in 1964 and the Sirikit Dambecame operational in 1974 after a 9-year construction pe-riod from 1963 to 1972 (Figure 1). It is likely that dam con-struction resulted in reduced sediment loading into the Gulfof Thailand.

Sedimentation in this area is highest in September andOctober when peak rainfall occurs (Figure 3). The sedimentpatterns or the upstream area of the Chao Phraya Riverbetween 1942 and 1988 (Figure 3) resembles the pattern ofrainfall distribution; however, there is likely to be an approx-imately 1-month time lag between rainfall and related sed-imentation. For example, the highest rainfall is recorded inSeptember, while the maximum sedimentation is recorded amonth later in October.

Severe wave attack is concentrated in the period fromMarch to the middle of May, although above-average waveaction continues until the end of September [15]. As strongerwave attack leads to increased damage to planted mangroves,special attention should be given to these months to protectmangroves from wave damage.

The effects of wind and/or wave attack on the coastare more severe if mangrove forests have been cleared. Theshoreline in the study region is retreating at a rate of 20 m y−1

[15], which is much higher than the figure of 3.6 m y−1 re-corded in the eastern part of the Thai Gulf [16]. According toa feasibility study conducted by SEATEC and BEST [17] on a14 km stretch of coastline in this area, the shoreline retreatedby 127 m, equivalent to 1,105 rai (1 rai = 40 × 40 m), duringthe 7-year period from 1987 to 1994, and retreated by 351 m(3,056 rai) during the 25-year period from 1969 to 1994.

2.3. Mangrove Plantation and Tree Measurements. A total ofapproximately 14,000 Rhizophora mucronata (RM) seedlingswere planted over an area of approximately 6 ha in September2001. Subsequently, tree height was measured on five occa-sions: June 2002, January 2003, June 2003, January 2005, andOctober 2005, representing periods of 9 months, 1 year and3 months, 1 year and 8 months, 3 years and 3 months, and4 years after planting, respectively. To monitor the plantedtrees, we designated three plots within the plantation site(Figure 4). Each plot contains 8 lines of 25 trees with a totalnumber of 200.

2.4. Soil Amendment Application. At sites of severe coastalerosion, it is important to induce the rapid growth of plantedmangroves, especially during the initial growth stages. To thisend, we applied three types of soil amendment: NPK, humicacid, and coconut fiber to 50 trees, respectively, at three plots.Coconut fiber is mixed into the seedling pot in Thai govern-mental mangrove nurseries to improve soil physical condi-tion. Mangrove ecosystems were identified as either nitrogen(N) or phosphorus (P) limited [18]; hence NPK was appliedin this study to enhance initial growth of mangroves. Con-tents of organic carbon influence initial growths of Rhi-zophora apiculata [13] so that humic acid as the fast-actingorganic carbon was applied. Analysis of variance (ANOVA)was applied to examine the difference of three types of soilamendment as well as to examine the effects of locality on

International Journal of Ecology 3

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Figure 4: View of the study site taken by remote-controlled helicopter from 500 m above sea level. The enclosed area shows the location ofthe study site (approximately 6 ha). Blue circles show the stations where the samples were taken for measurements of water, sediment, andbiological properties.

mangrove growth by the statistical software JMP 8.0.2.The statistical significance of differences was determined byANOVA followed by a multiple comparison test (Tukey-Kramer test) at 95% significance level (P < 0.05).

2.5. Water/Sediment Sampling and Biological Measurements.Water was collected from the study site (ST1) and an offshorestation (ST4) (Figure 4) and transported to the laboratorywhile kept chilled. The water samples were analyzed for tem-perature, dissolved oxygen, salinity, pH, ammoniacal-nitro-gen (NH+

4 plus NH3), nitrite-nitrogen, nitrate-nitrogen,total dissolved nitrogen (TDN), dissolved organic nitrogen(DON), particulate organic nitrogen (PON), total nitrogen(TN), dissolved inorganic phosphorus (DIP), total dissolvedphosphorus (TDP), dissolved organic phosphorus (DOP),particulate phosphorus (PP), total phosphorus (TP), partic-ulate organic carbon (POC), chlorophyll a, and biologicaloxygen demand (BOD). All of the above water analyses wereconducted according to the standard methods [19].

Sediment samples collected at the study site (ST1), anearby study site (ST2), the edge of shoreline (ST3), and anoffshore station (ST4) were analyzed for ammoniacal-nitro-gen, nitrite-nitrogen, nitrate-nitrogen, phosphorus, totalphosphorus (the sum of all forms of phosphorus), totalnitrogen, organic carbon, and acid volatile sulfide (AVS-S). Ammoniacal-nitrogen was measured by the phenolhypo-chlorite method [20]. Nitrate and nitrite were determinedtogether using the cadmium reduction method on a flowinjection analyzer (Lachat QuikChem 8000). Phosphoruswas analyzed using the spectrophotometric method [21]while total phosphorus was analyzed by ash/acid extraction[22]. Nitrogen and organic carbon were analyzed accordingto the high temperature combustion method using CHNelemental analyzer (LECO CHN-900) [23]. AVS-S was ana-lyzed from the surface sediment using H2S absorbentcolumns (GASTEC, Kanagawa, Japan) according to the pro-cedure described in Tsutsumi and Kikuchi (1983) [24]. Thetexture of each sediment sample was determined using thehydrometer method.

The abundance of plankton was measured from samplescollected in a 40 µm mesh plankton net and preserved in 5%formalin solution. The biomass of phytoplankton was stud-ied according to the enumeration method and chlorophyll acontent. Benthos abundance was calculated from the numberof benthic animals retained after passing the sample througha 0.5 mm benthos sieve; collected animals were preserved in10% formalin solution.

3. Results

3.1. Survival Rate and Tree Growth. The survival and growthrates of planted mangroves have varied since the mangroveswere planted in September 2001. In October 2005, 4 yearsafter planting, the survival rate decreased in the order ofPlot 1 > Plot 7 > Plot 9, with survival rates of 63.5%, 50.0%,and 26.7%, respectively (Figure 5). The survival rate variedwith distance from the shoreline and may correspond to theintensity of wave attack. As Plot 9 was located at the mostoffshore, it experienced the most severe erosion resultingfrom wind and wave action, thereby resulting in the lowestsurvival rate of the measured plots. Changes in the survivalrates for all plots were relatively minor over the first 3 yearsafter planting, but the survival rate for Plot 9 began todecrease markedly from January 2005, 3 years and 3 monthsafter planting. The survival rate for Plot 1 oddly increased inOct 2005 from that of Jan 2005, which is because the treesidentified as dead in Jan 2005 had revived in Oct 2005.

Tree heights measured 4 years after planting (October2005) were greatest in Plot 1, with an average height of308 cm. Mangrove showed the highest growths where thesurvival rate was highest, indicating that growth and survivalconditions were interrelated. Tree heights 4 years after plant-ing were greater in Plot 9 (248 cm ± 61.6 cm) than in Plot 7(214 cm± 96.7 cm), possibly due to different degree of wave/wind attack between the two plots. It is thus speculated thatphysical conditions were critical for the survival and growthof mangroves in the study region.

We also established an experimental Rhizophora plan-tation within an abandoned shrimp pond at Nakorn Sri


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Figure 5: Survival and growth of planted mangroves measured in June 2002, January 2003, Jun 2003, January 2005, and October 2005.Locations of the plots are shown Figure 4. Error bars denote standard deviation.

Thammarat, 700 km south of the present study site. Thesouthern site recorded a low frequency of tidal inundation,and tree heights measured 4 years after planting were around200 cm [13]. Accordingly, the average height of 308 cm meas-ured in the present study could indicate that growing con-ditions of the study site were relatively favorable in terms ofnutrient supply or local hydrology.

3.2. Soil Amendment Application. Plots to which the threetypes of soil amendment were applied showed higher growthrates than the unfertilized control plot (Figure 6). In ANOVAwe found significant difference in tree heights among thethree types of soil amendment at each measurement fromthe first (June 2002) to the last (October 2005) despite noapparent difference in the means (Figure 6). Effect of soilamendment on tree height was most significant in NPKtreatment at Plot 1 and Plot 9 while so in the coconut fibertreatment at Plot 7. Lower tree heights in Plot 7 than Plot 9(Figure 5) assured that growing condition was harsher in Plot7. Coconut fiber would have amended to stabilize erodiblesoils of Plot 7 which was more effective than NPK and humicacid.

If soil amendment effect with either NPK or coconut fiberwas noticed in the first measurement (June 2002), its effecthad continued to the subsequent measurements till the last

measurement (October 2005). This suggests that fertilizationof mangrove plants in early stage is important when they areplanted in eroded area. It appears that the effects of soilamendment are more pronounced in landward areas whereerosion is less severe. In terms of cost-effectiveness, fertiliza-tion therefore would be recommendable to be done first inlandward areas.

3.3. Water Quality. pH values were lowest at ST1, while salin-ity levels were almost similar at all sampling sites (Table 1).The low pH recorded at ST1 could have resulted from thehigh concentrations of POC at this site. Large amounts of or-ganic matter were produced by mangroves which were thendecomposed and consequently acted to lower the pH at ST1.

DON and PON were major nitrogen compounds asshown by higher concentrations than inorganic nitrogencompounds (ammoniacal-nitrogen, nitrite-nitrogen, and ni-trate-nitrogen). The concentrations of chlorophyll a rangedfrom 11.3 to 28.1 in ST1 and from 8.4 to 16.3 µg L−1 in ST4.Possible correlation between PON and chlorophyll a indi-cated that PON was originated from plankton [25]. Con-centrations of chlorophyll a fluctuated seasonally with thehigh concentration in April 2001 and with the relatively lowconcentration at other times.


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Figure 6: Effects of the fertilizer application on tree heights of June 2002, January 2003, Jun 2003, January 2005, and October 2005. Errorbars denote standard deviation. Different letters are significantly different at P < 0.05 by the Tukey-Kramer test.

BOD was high in all measurements, indicating that thestudy site was heavily polluted (Table 1). Tookwinas reportedthat the average BOD of effluent from shrimp ponds was8.47 mg L−1 [26]. At the mangrove plantation site (ST1),BOD was 4.3 mg L−1 when measured in August 2001, and4.9 mg L−1 in December 2001. These values, being approx-imately half of those for effluent from shrimp ponds, indi-cated that the mangrove plantation site has been subjected tothe inflow of polluted water from adjacent sites or from theTha Chin and Chao Phraya rivers (Figure 1).

The TN concentration at ST1 (1.9, 4.3 mg N L−1) washigher than that at ST4 (1.8, 2.7 mg N L−1), with these val-ues being higher than the average value for Kung KrabaenBay, Chantaburi province (0.4-0.5 mg N L−1) and lower thanvalues recorded from effluent water sampled from an inten-sive shrimp pond (4.9 mg N L−1) [26]. Values of TP, PP, andTDP were also higher at ST1, suggesting that the loadingswere derived from external sources. Similarly, as for nitroge-nous compounds, all phosphorous compounds (TDP, DOP,PP, TP, POC) also increased in December 2001 from April

2001, probably reflecting the high loading of effluent fromthe surroundings of the study site.

The levels of chlorophyll a at the plantation site (ST1)were higher than those in the offshore area (ST4), indicatingthat a primary producer (phytoplankton) was generated atthe plantation site. Levels of chlorophyll a showed temporalfluctuations, being especially high in April 2001 but relativelylow at other times. Levels of BOD varied between 4.3 and4.9 mg L−1 at the plantation site, probably reflecting the in-fluence of effluent from outside of the plantation site.

3.4. Sediment Quality. Nitrogen levels were relatively highin the mangrove plantation area. Ammoniacal-nitrogen wasdominant over nitrate-nitrogen and nitrite-nitrogen, indi-cating reducing conditions in the sediment (Table 2). Thesulfide content was relatively high in December 2001, reflect-ing the spontaneous input of organic matter from other sitesand the resulting increase in reducing conditions. Levels ofnitrate-nitrogen, which is the reduced form of nitrogen, also


Table 1: Results of water analyses conducted in April 2001, December 2001, and June 2002.

April 2001 December 2001 June 2002ST1 ST4 ST1 ST4 ST4

Water temperature (◦C) 30.4 31.1 27.0 28.0 32.6Dissolved oxygen (mg L−1) 3.8 4.5 7.1 5.4 3.8Salinity (ppt) 21.8 21.3 26.1 27.7 28.5pH 8.00 8.40 7.85 7.96 8.29Ammoniacal nitrogen (mg L−1) 0.012 0.021 1.005 0.014 0.010Nitrite-nitrogen (mg L−1) 0.008 0.028 0.199 0.014 0.005Nitrate-nitrogen (mg L−1) 0.009 0.012 0.024 0.005 0.028Total dissolved nitrogen (mg L−1) 0.607 0.691 2.540 1.165 0.375Dissolved organic nitrogen (mg L−1) 0.579 0.630 1.312 1.132 0.332Particulate organic nitrogen (mg L−1) 1.320 1.139 1.774 1.577 0.847Total nitrogen (mg L−1) 1.927 1.830 4.314 2.742 1.222Dissolved inorganic phosphorus (mg L−1) 0.091 0.086 0.425 0.150 0.029Total dissolved phosphorus (mg L−1) 0.107 0.086 0.852 0.819 0.078Dissolved organic phosphorus (mg L−1) 0.017 0.000 0.427 0.669 0.049Particulate phosphorus (mg L−1) 0.218 0.167 0.315 0.109 0.021Total phosphorus (mg L−1) 0.325 0.253 1.167 0.928 0.099Particulate organic carbon (mg L−1) 7.98 6.63 10.2 4.99 5.04Chlorophyll a (µg L−1) 28.1 16.3 11.3 8.44 10.2BOD (mg L−1: 5 days at 20◦C) 4.3 3.3 4.9 4.3 7.1

Table 2: Results of sediment analyses conducted in April 2001, December 2001, and June 2002.

Date StationTotal

nitrogen %

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Nitrite-nitrogenmg kg−1

Nitrate-nitrogenmg kg−1

Totalphosphorus

mg kg−1

Availablephosphorus

mg kg−1

Totalorganic

carbon %

AVS-Smg kg−1

April 2001

ST1 0.27 59.8 0.04 3.41 45.3 5.17 2.52 25.0ST2 0.30 112 0.06 4.20 44.6 18.1 2.70 77.3ST3 0.02 63.4 0.16 5.74 55.3 10.3 1.18 77.8ST4 0.04 47.0 0.04 3.25 86.9 8.79 0.97 214

December 2001

ST1 0.27 41.8 0.23 6.55 52.1 8.13 2.08 257ST2 0.31 91.9 0.33 8.36 80.4 27.2 2.36 213ST3 0.25 146 0.15 2.74 44.8 18.9 2.03 146ST4 0.26 42.1 0.15 2.46 88.7 10.4 2.02 92.1

June 2002

ST1 0.14 24.8 0.03 4.53 31.0 6.64 1.49 30.5ST2 0.20 25.9 0.00 5.11 35.7 9.57 2.06 51.4ST3 0.12 16.2 0.03 3.68 16.2 13.8 1.47 57.7ST4 0.15 20.6 0.11 2.77 30.2 2.17 1.97 134

increased at this time. Constantly high levels of sulfide at ST4indicate large amounts of organic matter, even in the offshoreareas.

In June 2002, reducing conditions were measured in theoffshore area (ST4), probably due to effluent from majorrivers. Considering that typical coastal sediment in Thailandcontains 1.06% organic carbon and 0.05% total nitrogen[26], the mangrove plantation site (including the offshorearea) experienced a high degree of eutrophication at thistime. Available P was higher at ST2 than at ST1. Since ST2 islocated in the middle of a small creek, we consider that phos-phorus was transported along the creek from offshore areas.

3.5. Benthos. The abundance and species composition ofbenthos varied considerably at each measurement period;

however, values were generally higher during the monsoonseason (April, June) than during the dry season (December)(Table 3). Environmental factors such as sediment compo-sition [27] and salinity [28] affected the benthic community.Sediment texture coarsened in the order of ST1<ST2<ST3<ST4 (Table 4); the number of species and abundance alsoincreased in this order (Table 3). The diversity of Polychaetahas been reported to be related to the degree of salinity of thehost sediment [29, 30]. In this study, Polychaeta was dom-inant at ST1 during the monsoon season when the influenceof salinity was relatively minor while the diversity of Crus-tacea increased with lower salinity.

3.6. Phytoplankton. We identified 3 groups and 33 generaof plankton (Table 5). The blue-green algae group yielded


Table 3: Species of benthic fauna and abundance (individuals/m2) within sediment and bottom of the replanted mangrove areas.

April 2001 December 2001 June 2002

No. Group ST1 ST4 ST1 ST2 ST3 ST4 ST3 ST4

1 Balanus 133

Crustacea 2 Leucosia 44

3 Nassarius 30

4 Uca spp. 15

Gastopoda1 Littorina 89 15

2 Paludinella 30

1 Anadana 44

2 Mactra 178 134

Pelecypoda 3 Musculus 222

4 Mytella 89

5 Pholas spp. 2,682

6 Tellina 89 533 15 30 134

7 Tellina foliacea 15

1 Aquilaspio 133

2 Cerithium 44

3 Glycera 267

4 Eunic 133 1,689 400

5 Nephytys 356

Polychaeta 6 Nereis 504 445 1,067 1,600

7 Notomastus 578

8 Polyodontes 222

9 Sabella 267

10 Sigambra 667

Table 4: Results of texture analysis.

Station Clay (%) Silt (%) Sand (%) Type

ST1 49.7 19.3 31.1 Clay

ST2 35.1 27.6 37.3 Clay loam

ST3 28.8 31.2 40.1 Clay loam

ST4 14.2 29.3 56.5 Sandy loam

5 genera, green algae 3, and diatoms 25. Except for ST1 inApril 2001, diatoms were the dominant group in terms ofthe number of species and the number of genera. Diatomsbloomed at the low rainfall periods (December 2001), withthe increased number of genera to 12 genera and of thetotal cell number to 62,837 cells. At the most offshore site(ST4), the number of phytoplankton genera and of totalphytoplankton cells increased in the order of April 2001(9 genera, 3,164 cell L−1), December 2001 (13 genera,66,922 cell L−1), and June 2002 (15 genera, 33,201 cell L−1).This trend appears to correspond to the salinity level: a great-er number of genera occurred during the periods of highersalinity. Similar trends have been reported for diatoms (thegreater number of genera during the periods of low precip-itation and at the sites with the higher salinity) in SongkhlaLake, Thailand [31].

The species composition of phytoplankton was related tothe water quality. In December 2001, the water was eutroph-icated as reflected by the higher contents of phosphorus andnitrogen (Table 1). Under these conditions, diatoms becamedominant at the expense of other groups.

4. Discussion

Nitrogen fertilization is reported to increase growth rates ofmangroves which were planted in an abandoned impound-ment in Florida [32] while our study showed that fertilizationwas also effective in the open area with high-energy environ-ments.

Establishment of mangroves in heavily eroded environ-ments has been difficult. Total area losses accounted for0.91 km2 y−1 for the Thai Gulf coast. However the presenceof mangroves reduces the erosion rates in areas where erosionprevailed [33, 34]. Enhancement of initial mangrove growththrough soil amendment therefore should be an effective wayto reduce erosion rates at heavily eroded places. The effectsof soil amendment are limited if the plantation site is heavilyeroded, as the case of Plot 7 in the present study. This suggeststhat the planting of mangroves along eroded shorelinesshould be initiated within the least erosion-prone areas.

Physical measures can be undertaken to protect seedlingsfrom wave/wind attack; however, this approach is ineffective


Table 5: Results of phytoplankton (cell L−1) measurement conducted in April 2001, December 2001, and June 2002.

Density of phytoplankton (cell L−1)

April 2001 December 2001 June 2002

Phytoplankton group No. Genera ST1 ST4 ST1 ST4 ST4

1 Anabaenopsis 397

2 Oscillatoria 1,008 336 225

Blue green algae 3 Gloeocapsa 14,463

4 Lyngbya 140

5 Spirulina 117 24

1 Gonatozygon 332

Green algae 2 Schroederia 91 14

3 Staurastrum 117

1 Skeletonema 2,581 462 7,125

2 Coscinodiscus 11,427 742 827 22 18

3 Dactyliosolen 11 33

4 Rhizosolenia 468 882 30,645 8,974 1,410

5 Bacteriastrum 33 13,688

6 Chaetoceros 273 7,425 22,000 8,250

7 Odontella 281 28 51

8 Thalassionema 6,210 72 1,185

9 Gyrosigma 146 27

10 Navicula 163 252 11 44

11 Nitzschia 345

12 Pseudonitzschia 17,145 35,132

Diatoms 13 Campylodiscus 11

14 Dictyocha 3

15 Noctiluca 1,050

16 Ceratium 105

17 Alexandrium 6

18 Fragilaria 137 238

19 Eucampia 11 6

20 Melosira 245

21 Hemiaulus 11 44

22 Pleurosigma 124 98 114

23 Pseudosolenia 550

24 Thalassiosira 6

25 Stephanodiscus 98

in most cases. We erected bamboo fencelines at other studysites to protect mangrove seedlings from wave/wind attack,but the fences lasted no more than 1 year. The constructionof physical barriers is also extremely expensive. According tothe Japan International Cooperation Agency (JICA) study,the planting of mangroves across the 4,800 m shoreline of thepresent study area using civil engineering techniques to pre-vent coastal erosion will cost 983 million baht (approximate-ly 31 million US dollars as of July 2011) [15]. In Malaysia,a 90 m long rubble mound structure (breakwater) was con-structed along an eroding tropical shoreline at a cost of US$42,850 which is more than 10 times lower [35]. This kind ofcost is quite variable according to the sort of construction

method which is adopted. However civil engineering opera-tion for coastal protection is generally rather expensive.

The disastrous tsunami that hit the Thailand coast inDecember 2004 revealed the important role mangroves playin terms of coastal protection. It appears that areas coveredin mangroves suffered less damage than mangrove-free areas,and the protective effect of mangroves against tsunami dam-age differed among different species. Kamali and Hashim(2011) reported that Rhizophora spp. were found to have agreater protective effect than Sonneratia or Avicennia spp.[35]. These differences are attributed to root architecture, asthe prop roots of Rhizophora spp. are stronger than the cableroots of Sonneratia and Avicennia spp.


As any reduction in sediment supply from the majorrivers is likely to influence the retreat of the coastline in thestudy region, complete protection against coastal erosion isnot possible; however, the planting of mangroves is a poten-tially effective measure in lowering the present erosion rate.Mangrove-planting techniques should therefore be devel-oped to cope with coastal erosion, especially in terms of ade-quate species selection, site identification, and additionalmeasures such as soil amendment application.

The study site was highly polluted as indicated by water,sediment, and biological measurements, which could beinfluenced by the surrounding areas. Mangrove soils have alarge buffering capacity for pollutants [36]. However, its ca-pacity has a limit. With excessive loadings, mangrove soil canno longer retain pollutants, and consequently marine ecol-ogy and fishery resources will be severely damaged. Consid-ering the pollution level of the study area, it should be neces-sary to watch carefully a status of pollution level of mangrovearea in order to prevent drastic marine pollution.

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